Continuous conveying process and device for shear-sensitive fluids

ABSTRACT

The invention relates to a process for continuously conveying shear-sensitive fluids, in particular polymer latices or plastics dispersions, which is accompanied by the taking of samples and the processing and analysis thereof, to a device for implementing the process and to a new double-long-stroke reciprocating pump.

[0001] The invention relates to a process for continuously conveyingshear-sensitive fluids, in particular polymer latices or plasticsdispersions, a device for implementing the process and also a newdouble-long-stroke reciprocating pump.

[0002] It is known that when subject to shear—when being conveyed forexample—polymer latices or plastics dispersions can easily coagulate—ie,from the finely dispersed fluids solid substances (coagulate) areprecipitated which can coat or obstruct conveying elements andpipelines. This coagulate can also be deposited on metering probes ofall types which are immersed in the latex or which come into contactwith latex and can falsify or hinder ongoing measurements. Laticeswhich, with a view to the quality of the product, have to be producedwith a minimum of emulsifier are in particular highly prone to formcoagulate.

[0003] Known conveying devices are rotary, diaphragm or reciprocatingpumps. Rotary pumps consist of stator (casing) and rotor (runningwheel). As a result of the high speed of rotation of the running wheelthe product to be conveyed is accelerated radially from the centre ofthe rotor (central point) and pressed through the pressure connection ofthe casing by the corresponding centrifugal forces at the outer diameterof the running wheel. As a rule, rotary pumps require a speed ofrotation of more then 500 rpm. At a lower speed no product is conveyedwith this system. By virtue of their design characteristics rotary pumpshave large clearance volumes and are not self-priming.

[0004] Diaphragm pumps and reciprocating pumps are displacement pumpswhich operate as rotary pumps at lower frequencies. The lowest frequencyis around 30 strokes/min. The conveying motion of the piston or of thediaphragm, which is also moved on the drive side by a piston, isintermittent—pulsations occur, so that, considered in the short term, itis not possible to speak of a continuous conveying or transport ofproduct. The pulsation is pressure-dependent and has an effect on theconstancy of conveying and the dosing precision.

[0005] Long-stroke reciprocating pumps are known as single-strokereciprocating pumps, but they are unsuitable and too expensive for thecontinuous dosing of small quantities (see Menges, Recker,Carl-Hanser-Verlag; Munich, Vienna 1986, Automatisieren in derKunststoffverarbeitung, pages 318-320).

[0006] A disadvantage with these conveying implements is that, forexample, rotary pumps can only convey at a high speed of rotation andcan build up pressure. As a result of the high circumferentialvelocities on the rotor, product particles are very strongly sheared atthe gaps in the pump head, so that the particles undergo a change. Owingto a high clearance volume a rotary pump is unsuitable for conveyingsmall quantities of product with subsequent sampling. For example, it isnot possible to withdraw a representative sample quantity of, forexample, 10 ml from a reactor and to make inferences therefrom as toreaction conditions in the actual reactor. Piston pumps or diaphragmpumps operate at stroke frequencies that are too high, and by virtue oftheir design characteristics they have many narrow gaps in whichshearing of product or particles occurs. An additional factor whichalters the product is the inconstant speed of the displacement elements.On pipelines or pump-head parts that have been touched by product theirregular delivery rate brings about various frictional forces whichcause product shearing in the microparticle range. With the known pumpdesigns the dwell-time of the product in the pump head is not finite ornot precisely defined, as all pump heads have a large clearance volume.Large proportions of a suction-stroke volume remain in the pump head fora long time, with each new suction stroke only a partial mixing with oldproduct occurs, so that product particles are subjected to shear stressover a long period. A further disadvantage is that the piston pumps ordiaphragm pumps have no positive valves (ball valves) which, dependingon suction or compression process, open a large, free cross-section. Thenon-synchronicity of suction process and compression process has afurther disadvantageous effect. With these pump designs an absolutesynchronous operation of suction side and compression side is notpossible, even if an improvement in the conveying can be partiallyachieved by means of multiple heads. By virtue of the rotationalacceleration of the control gearing and the associated conversion ofrotary motion into reciprocating motion, the individual processes thathave to be considered in the individual pump head are always irregularand are the reason for the primary pulsation in the flow beingtransported.

[0007] A further disadvantage with known pump designs is that as thevolumetric flow becomes smaller the clearance volume in the pump headbecomes greater. This means that the actual conveying displacement bodychanges its working stroke in proportion to the flow rate.

[0008] Single-piston pumps are of very large construction, are veryexpensive and do not work continuously. The designs do not permitself-priming so that pumps are required in addition on the suction side.Their large clearance volume makes them unsuitable for sampling, forexample. Even in the case of twin-piston reciprocating pumps that areconceivable, synchronous operation of suction and compression processesis not possible.

[0009] The object underlying the invention is to convey shear-sensitivefluids, in particular polymer dispersions, in continuous, low-pulsationand gentle manner so that the fluids remain unaffected in their phasecondition by the conveying, and in particular in the case of dispersionsno phase separation or coagulation occurs. In particular it is intendedto make it possible to convey a polymer latex out of a reactor into arecirculating loop and back, whereby the formation of coagulate or there-forming of particles is avoided. The object further underlying theinvention is to make available a device which enables this conveying ina manner which is gentle and which over a long period is trouble-freeand operationally dependable. In particular, the device should make itpossible to transport the fluid back and forth in gentle manner betweenvarious parts of the plant, for example a reactor and a metering loopwith differing pressure conditions. This object is achieved inaccordance with the invention in that the fluid is aspirated and pumpedby means of a double-long-stroke reciprocating pump which isclearance-free and has low pulsation and that, in the system of pipesthat is flowed through, use is made of clearance-free valves. Thisdouble-long-stroke reciprocating pump is located, for example, in apressure loop, whereby through a sluice or an overflow valve whichserves as connecting element to a metering circuit with a differentpressure level arranged parallel, a defined sample quantity istransported and is supplied to the actual on-line gauges in the parallelmetering circuit by a second long-stroke reciprocating pump of identicaldesign.

[0010] The invention provides a process for continuously conveyingshear-sensitive fluids, in particular polymer latices or plasticsdispersions, having a viscosity of up to 100,000 mPa.s with a pumpcapacity of 10 ml/h to 100 l/h, characterised in that the fluid isaspirated via at least one clearance-free valve by a piston of adouble-long-stroke reciprocating pump, while fluid is synchronouslyemitted from the second piston chamber likewise via at least oneadditional clearance-free valve towards the conveying side and aftertotal evacuation of the second piston chamber the first valve is openedon the emission side and is closed on the suction side while the secondvalve is closed on the emission side and is opened on the suction sideand the direction of motion of the pistons is synchronously reversed. Apreferred embodiment is characterised in that the fluid is conveyed in arecirculating loop which is coupled to a reactor. Within thisrecirculating loop, or parallel to the latter, metering probes orsensors or built-in assemblies may be located. Examples of such meteringprobes are temperature sensors, pH-electrodes, conductivity electrodes,NIR light-guide probes, oscillating U-tubes for density measurements,refractometers, ultrasound metering heads or devices for calorimetry.The stated metering probes or gauges are not coated or obstructed by thefluid which is conducted in the circuit (eg, a latex). In principle itis possible to introduce into the recirculating loop additional devicesfor thorough mixing, such as static mixers or heat exchangers which, byvirtue of the conveying device which is constantly pulsation-free andwhich exerts a low degree of shear, are not coated or obstructed. Afurther preferred variant of the process is characterised in that thefluid is conducted through an overflow valve or a sluice into a regionof reduced pressure (eg, a secondary recirculating loop). Particularlypreferred is a variant of this process in which the overflow valve orthe sluice is connected by way of coupling between primary and secondaryrecirculating circuits. With this variant it is possible to branchdefined sample volumes of the fluid out of the principal-flow line or aprimary recirculating circuit and, for example, to cause it to bemetered under reduced pressure.

[0011] The invention is elucidated in greater detail below withreference to an example of an embodiment represented in the drawings.

[0012] Shown are:

[0013]FIG. 1 a scheme of the conveying, in accordance with theinvention, of shear-sensitive fluids

[0014]FIG. 2 a schematic illustration of the conveying process as partof a recirculating loop and for decoupling polymer

[0015]FIG. 3 structure of the double-long-stroke reciprocating pumpaccording to the invention in a side view

[0016]FIG. 4 a double-long-stroke reciprocating pump according to theinvention in top view.

[0017] Connected to a reactor 1 for the emulsion polymerisation ofbutadiene which operates under a pressure of >5 bar is a recirculatingloop 2 which contains a new double-long-stroke reciprocating pump 3 withpositive input and output valves 4, 5 as conveying device and also anoverflow valve 6 which connects the primary circuit 2 to the secondarycircuit 7. The newly developed double-long-stroke reciprocating pump isillustrated in FIGS. 3 and 4. Its two pistons 8 and 9 are driven via anangle-stroke gearing 10 with control gearing 11 connected upstream. Thestroke volume of the pump heads can be adjusted by means of a ring 12which simultaneously fulfils the function of actuating a contact switch13 for changing the direction of rotation and for the change-over of thearmatures. Double seals and support rings are placed on the head of eachpiston in such a manner as to seal in relation to the casing. The headseal of the pistons enables the formation of a low-clearance pump headirrespective of the extent of piston displacement. The product volumeaspirated is displaced quantitatively out of the pump head during theconveying. While on the one hand the reaction mixture is, for example,slowly aspirated by the piston 23, the opposite piston 24, which isguided via the same spindle 14 as the piston 23, pushes previouslyaspirated reaction mixture quantitatively out of the pump head. Thedouble-long-stroke reciprocating pump according to the invention isself-priming and self-ventilating at a pulsation frequency of less than10 strokes per minute. The clearance of the pump amounts to less than 1%of the pump-head volume. It is possible to work with the pump at apressure of up to 300 bar and a temperature of −100 to +125° C. With theaid of the pump according to the invention it is also possible forfluids containing solid to be pumped if the sedimentation-time of thesolid is greater than the dwell-time of the fluid in the pump head. In apreferred embodiment the spindle of the pump has an additionalanti-torsion device.

[0018] The double-long-stroke reciprocating pump 3 makes it possible torecirculate a partial-flow quantity of the reaction volume from thereactor 1 in a manner which at no time harms the product. With the aidof the device according to the invention 100 ml/h of butadiene polymerwere recirculated by pumping for over 100 h at a pressure of 5 bar and atemperature of 50° C. without sedimentation or coagulation.

[0019] In a preferred embodiment the recirculating loop 2 is connectedto a vacuum vessel 13 via an overflow valve 6. The overflow valve 6prevents a spontaneous expansion of the liquid monomers contained in thesample quantity. As a result, an uncontrolled formation of foam isprevented. The vacuum vessel 13 has a defined volume and is evacuatedvia a control system to a preselected reduced pressure, for example 50mbar. Once the reduced pressure has been reached, the control systemswitches a valve 25 located in the recirculating loop into the closedposition so that the double-long-stroke reciprocating pump 3 pumps thedefined volume against the valve 25 that is in the closed position andincreases the system pressure within the recirculating loop. Theoverflow valve 6 allows the sample to pass into the vacuum vessel at apreviously adjusted pressure which is above the reactor pressure. Assoon as the sample has been channelled out of the recirculating loop thecontrol system gives the command to open the valve 25 so that therecirculating circuit is again put into operation. The injected samplegenerates an increase in pressure in the vacuum vessel, which consistsof a calibrated, cylindrical metering vessel 13 and an equalising vessel15. The equalising vessel is preferably so designed that upon expansionof a low-boiling component of the multi-phase fluid the maximum pressurearising definitely does not exceed 1 bar. The increase in pressure is inturn intensified by components of the sample which evaporate. If after acertain time (eg, <30 min) the pressure in the metering vessel no longerchanges, then the pressure difference is calculated and together withthe temperature, the volume of the sample and the volume of the vacuumvessel the monomer concentration is determined and an inference isthereby made as to the present composition of the product in thereactor. If the pressure-generating component of the reactor sample isisolated, the remaining, unevaporated sample is automatically comparedwith the prespecified target sample quantity. If the metered samplevolume is below the target value, the vacuum vessel is again evacuatedand a further sample is required from the double-long-strokereciprocating pump. This partial process is repeated until the samplequantity is sufficient. Then the vacuum vessel 13 is ventilated withinert gas and the rest of the sample is pumped into a metering loop. Themetering circuit 7 is equipped with a single-long-stroke reciprocatingpump 16 in order to provide metering probes with product. Thesingle-long-stroke reciprocating pump is equipped, like thedouble-long-stroke reciprocating pump, with positive valves 17 and 18.Once the isolated sample has been aspirated out of the vacuum vessel,the valves 17, 18 switch over to the actual metering circuit. As aresult, the vacuum vessel between valve 6 and valve 17 is temporarilyexcluded from the remaining process. Now a process for cleaning thedegassing cell can proceed automatically, consisting of flushing anddrying processes in parallel with the other automated process. In thecase of sample-processing operations upstream the flushing process isnecessary in order to clean parts that have been moistened with product,in order that no falsification of measured values can occur insubsequent measurements. The flushing and drying processes are initiatedvia the valves 20 and 21 respectively. After the flushing liquid hasbeen introduced into the vacuum vessel the valve 22 opens in order toallow the quantity of flushing medium that has been channelled in todrain off into a coupled collecting vessel. The flushing process mayoptionally be repeated several times depending on productcharacteristics. After the flushing is over, a drying process takesplace which is initiated via the valve 21. The drying process is onlyrequired when the remaining quantity of flushing liquid that adheres toinner walls does not evaporate as a result of the subsequent evacuationand thereby falsifies the initial measured value for the determinationof the low-boiling component. It is possible for further analytical andmetering devices to be installed within this metering loop in order todetermine different properties of the degassed sample.

[0020] The metered sample remainder can be supplied from the meteringloop into the recirculating loop 2 via an additional three-way valve andthereby conducted back into the reactor as seed polymer. The conveyingof the polymer within the metering loop is preferably also carried outwith the aid of an additional conveying device according to theinvention, in particular with the aid of a second pump according to theinvention. With the aid of the process according to the invention it ispossible to achieve a fully automatic sampling and determination of thecurrent monomer concentration during a pressure polymerisation by meansof the stated pressure-difference measurement. The monomer concentrationdetermined in this connection can be used by way of control variable forthe dosing of added monomer or initiator. It proves to be a particularadvantage that by virtue of the gentle conveying and the furthertreatment, within the metering loop for example, the metered polymer canbe supplied again to the reaction mixture in the reactor withoutimpairing the quality of the product. A variant with a view to thetransport of polymer into the metering loop consists in sealing off adefined volume of the recirculating loop 2 which is filled with reactionmixture and at the same instant opening a loop bypass so that theproduct flow within the recirculating loop is not interrupted. Thequantity of reaction mixture under pressure in the sluice is thenspontaneously discharged into the vacuum vessel. Then the sluice isswitched into the flow of reaction mixture again and the loop bypass isclosed. In this connection the sluice replaces the overflow valve 6.

[0021] It is possible to complete the demonomerisation or processing ofthe reaction mixture in the vacuum vessel by repeated evacuation andsubsequent ventilating with inert gas, whereby in the case of foamingproducts, for example, light barriers can be used as foaming guards.

[0022] Typical measurements which are made, for example, ondemonomerised latex within the course of the metering loop are density,refractive-index, NIR, ultrasound, pH and conductivity measurements.Furthermore, measurements of light extinction (where appropriate atvarious wavelengths) or a determination of particle size with the aid oflaser correlation spectroscopy can be carried out on demonomerised latexwhich has been diluted in defined manner. By means of a combination ofthe data from these measurements it becomes possible to determine thekinetics of the polymerisation while the reaction is in progress and toimplement a control of the process.

[0023] The invention further provides a low-pulsation, clearance-free,double-long-stroke reciprocating pump with a pump capacity of 10 ml/h to100 l/h for the conveying of shear-sensitive fluids having a viscosityof up to 100,000 mPa.s, exhibiting two pistons 8, 9 on a common drivespindle 14, an angle-stroke gearing 10 with control gearing 11 connectedupstream or a hydraulic gearing for driving the pistons 8, 9, a ring 12for adjusting the stroke volume of the pump heads 23, 24, a contactswitch 26 for changing the direction of rotation of an initiator disc 27and also double seals on the head of the piston chambers.

[0024] The pump is driven, for example, via a reduction gearing withangle-stroke gearing connected in series which converts the rotarymotion into rotation-free reciprocating motion of the pistons 8, 9. Thishas the advantage that the suction-piston and compression-piston headsare mounted on one spindle and the double reciprocating pump can conveycontinuously. By means of this arrangement, compression piston andsuction piston run in absolutely synchronous manner. In each case theside of the respective suction-piston head facing away from the producttriggers the switch in order to change over from the compression processto the suction process. The design may also be such that the pressurehead, rather than the suction head, triggers the change-over. In thiscase the switching sensor would have to be placed on the pump-head siderather than on the gearing side. The reciprocating rod, which is athreaded rod (piston rod) on which the two piston heads are located, isfor example provided with a groove extending axially, with which ananti-torsion device preferably engages in order to avoid rotation of thereciprocating rod. The piston head is provided with at least one elasticseal with respect to the piston casing and at least one guide ring sothat the aspirated product can also be displaced totally out of thepiston-head casing in the course of the pressure process.

[0025] Suction and compression chambers are, for example, connected toone another via two capillaries in which two positive three-way valves(ball valves) (4, 5) are placed. The three-way valves provide for aseparation of the two chambers. Instead of the two three-way valves,four single valves can also be provided. The valves preferably consistof ball valves in order to ensure a shear-free passage of product. Eachpiston has a product inlet opening and a product outlet opening arrangedvertically above one another, the inlet at the bottom and the outlet atthe top, in order to enable self-ventilating and self-priming of thepump. In a particular embodiment a telescopic spindle (length-adjustablespindle) is employed in order to be able, in the case of extremely smallpiston displacements, to generate a pump circulation and to keep theresidual volume low. The double-long-stroke reciprocating pump candischarge the pump head in remainder-free manner irrespective of theflow rate. By reason of the extremely low stroke frequency (eg, max. 2strokes per minute), a low-pulsation conveying of product takes place,the product is conveyed in laminar manner, a plug flow occurs in thecapillaries or pipelines. At constant pressure the laminar conveying ofa product is a prerequisite for the shear-free transport of latices. Thepump drive and the pump-head volume should preferably be so designedthat a switching cycle of greater than 5 minutes is maintained. As aresult, shear-free pumping, of the polymer latices or plasticsdispersions for example, is ensured. These frequency characteristicsdescribe a pulsation-free (low-pulsation) conveying system which canpump shear-sensitive products over a long period.

[0026] By virtue of its simple design the pump can convey continuouslyextremely small quantities, from 10 ml/h, or can also aspirate definedquantities in the ml-range and can be used as sampling apparatus withdiscontinuous or continuous processes. The pump can function as asluice, since compression side and suction side are separated from oneanother, it can transport product quantities from a region of lowpressure or a region of excess pressure into a region of low pressure ora region of excess pressure respectively. By reason of theclearance-free design, no back-mixing occurs with temporally olderproduct quantities.

[0027] A particular advantage of the process with the double-long-strokereciprocating pump is that with the adjusted sample quantity of, forexample, 10 ml a defined product quantity per stroke is conveyed and canbe transported at any time into, for example, a metering circuit. Thissample quantity or the maximal stroke distance is capable of beingadjusted with an adjusting nut on the piston spindle inside the pistoncasing.

[0028] The piston heads and pump casing may consist of suitable metallicand/or non-metallic materials. In special embodiments the piston casingmay also be lined with glass or ceramic sleeves. The piston casing canbe heated or cooled in simple manner.

[0029] By virtue of the separation of compression side and suction side,arbitrary positive or negative pressure differences can exist betweenthese sides. The pump can be operated at a temperature of −100° C. to+200° C. It is usual to employ the pump in a temperature range of about−20 to +150° C., in the case of latices in the range from +2 to +100° C.The ratio of stroke volume to residual volume in the pump head, which isa measure of the clearance freedom, preferably amounts to less than 1%.

1. Process for continuously conveying shear-sensitive fluids having aviscosity of up to 100,000 mPa.s with a pump capacity of 10 ml/h to 100l/h, characterised in that the fluid is aspirated via at least oneclearance-free, positive valve (4) by a piston (8) of adouble-long-stroke reciprocating pump (3) while fluid is synchronouslyemitted by the second piston (9) from the second piston chamber (24)likewise via at least one additional clearance-free, positive valve (5)towards the conveying side and after total evacuation of the secondpiston chamber (24) the valve (4) is opened on the emission side andclosed on the suction side while valve (5) is closed on the emissionside and opened on the suction side and the direction of motion of thepistons (8, 9) is synchronously reversed, whereby the pistons (8) and(9) are located on a spindle (14).
 2. Process according to claim 1,characterised in that the shear-sensitive fluid is conveyed in arecirculating loop (2) which is coupled to a reactor (1).
 3. Processaccording to claims 1 and 2, characterised in that the shear-sensitivefluid is a polymer latex or a plastics dispersion.
 4. Process accordingto claims 2 and 3, characterised in that the fluid is pumped into theregion of reduced pressure via an overflow valve (6) or a sluice whichis connected between the recirculating circuit (2) and a region ofreduced pressure.
 5. Process according to claim 3 to 4, characterised ithat the region of reduced pressure is part of a metering loop (7) whichis parallel to the recirculating loop (2) and by way of metering devicesexhibits at least one calibrated vacuum vessel with pressure gauge fordetermining monomer concentration of the polymer latex.
 6. Device forimplementing the process according to claim 1, exhibiting adouble-long-stroke reciprocating pump (3), at least one clearance-free,three-way valve (4) which is positively controlled via a contact switch(26) of the double-long-stroke reciprocating pump (3) and at least oneclearance free, three-way output valve (5) which is likewise positivelycontrolled via a contact switch (26) of the double-long-strokereciprocating pump (3).
 7. Device according to claim 6, characterised inthat it is connected to a recirculating loop (2) which is coupled to areactor (1).
 8. Device according to claim 7, with an overflow valve (6)or a sluice on the emission side of the conveying device in therecirculating loop for transporting fluid into a region of reducedpressure.
 9. Low-pulsation, clearance-free, double-long-strokereciprocating pump with a pump capacity of 10 ml/h to 100 l/h forconveying shear-sensitive fluids having a viscosity of up to 100,000mPa.s, exhibiting two pistons (8, 9) on a common drive spindle (14), anangle-stroke gearing (10) with control gearing (11) connected upstreamfor driving the pistons (8, 9), a ring (12) for adjusting the strokevolume of the pump heads (23, 24), a contact switch (26) for changingthe direction of rotation, an initiator disc (27) and also double sealson the head of the pistons (23, 24).
 10. Double-long-strokereciprocating pump according to claim 9, characterised in that thespindle (14) exhibits an anti-torsion device.
 11. Double-long-strokereciprocating pump according to claims 9 and 10, characterised in thatthe spindle (14) is a telescopic spindle and replaces the adjusting nut(12).
 12. Double-long-stroke reciprocating pump according to claims 9 to11, characterised in that the movement of the piston is controlled via ahydraulic system instead of the angle-stroke gearing (10) with controlgearing (11) connected upstream.